1. Field of Invention
The present invention relates to an advanced treatment method for water.
2. Description of Related Arts
Recent industrial and agricultural developments have led to discharges of a large amount of industrial wastewater and domestic sewage into rivers, lakes and reservoirs. While these rivers, lakes and reservoirs are usually the water source of domestic water supply for local inhabitants, the wastewater discharge has aggravated the problem of organic pollution in the water source of drinking water. Some organic pollutants, such as chemical substances which include chemical raw materials, pesticides and plasticizers, are carcinogenic, teratogenic and mutagenic. The concentration of these substances is generally low in water but the presence of these substances is extremely harmful to health because of their high level of toxicity. Conventional water treatment methods have very limited effect on the removal of these substances. For example, in Nov. 13, 2005, an accident occurred in Jilin, China. A diphenyl factory owned by China National Petroleum Corporation exploded and led to a massive nitrobenzene pollution in the Songhua River and the emergency use of activated carbon powder. This activated carbon powder provided strengthened adsorption for coagulation so that the emergency use of activated carbon powder is required to ensure the safety of drinking water because the conventional water treatment has very poor nitrobenzene removal effect on water. However, activated carbon has a problem of saturated adsorption capacity and has to be regenerated after its adsorption capacity is saturated. Hence, the cost of the use of activated carbon powder is relatively high. For removal of this type of organic substances, ozonation is another possible method. However, because of the limited ability of ozonation in which only a selective portion of the easily oxidized organic substances containing benzene ring or double bond structure can be removed, the removal of organic pollutants which are highly stable and hard-to-degrade are very difficult to achieve to the extent of complete mineralization. In order to enhance the effect of ozone on the removal of organic pollutants in water, catalytic ozonation is employed. It is generally accepted that this method can effectively remove the organic pollutants in water by promoting the decomposition of ozone in water and producing strong oxidizing hydroxyl radicals. Common catalytic processes include homogeneous catalytic ozonation which utilizes metal ions as the catalyst and heterogeneous catalytic ozonation which utilizes metal oxides and supported noble metals as the catalyst. However, the common catalytic processes also have the following drawbacks. For homogeneous catalytic ozonation, the metal ions are dissolved in water and are lost with the water flow, therefore causing secondary pollution. For heterogeneous catalytic ozonation, the metal oxides are usually in powder form and have to be supported by other materials in order to prevent loss with water flow, therefore complicated manufacturing process and higher manufacture cost are involved while the problems of low catalytic efficiency and dissolution of metal ions have adversely affected the water quality and treatment result. The supported noble metal catalyst involves high manufacturing cost and is not suitable for large-scale application. At present, there is also water treatment method for removal of organic pollutants by a combination use of metal and ozone. For example, in Chinese patent application number 20081006448.7, a method of removing organic substance in water by catalytic ozonation was disclosed in which the catalyst makes use of a mixture of zerovalent iron and filler material. However, the catalytic ozonation process produces ferric ion which causes the treated water to turn yellowish, hence adversely affects the sensory properties of water and produces poor water treatment result.